This invention relates to a process for converting a paraffinic feed containing a major proportion of C.sub.2 to C.sub.12 aliphatic hydrocarbons, e.g., a Udex raffinate, to aromatics, or mixture of aromatics and olefins, in the presence of a zeolite catalyst.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (Reissue U.S. Pat. No. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.
Zeolites and alumina have been used in the past in the preparation of catalysts for the production of aromatic hydrocarbons from aliphatic hydrocarbons. The aliphatic hydrocarbon is passed over the catalyst at an elevated temperature in the liquid or vapor phase. Zeolites of various types have been suggested for the preparation of such catalysts. Examples of such zeolites are mordenite and ZSM-5.
U.S. Pat. No. 3,756,942 discloses a process for converting paraffinic feedstocks over zeolites such as ZSM-5 to produce a variety of hydrocarbon products. The underlying chemistry involved in this conversion is extremely complex. More particularly, a number of simultaneous and sometimes competing reactions take place to produce a variety of products which can, in turn, be reacted to form still different products. These possible reactions include cracking of paraffins, aromatization of olefins, and alkylation and dealkylation of aromatics. Products from the conversion of C.sub.5.sup.+ paraffinic feedstocks over ZSM-5 include C.sub.6 -C.sub.8 aromatics, C.sub.2 -C.sub.4 olefins, C.sub.9.sup.+ aromatics and C.sub.1 -C.sub.3 paraffins. Of these products the C.sub.6 -C.sub.8 aromatics and C.sub.2 -C.sub.4 olefins are the most desired.
C.sub.6 -C.sub.8 aromatics, e.g., benzene, toluene, xylene and ethyloenzene, also referred to collectively as BTX, are valuable organic chemicals which can be used in a variety of ways. Since BTX has a high octane value it can be used as a blending stock for making high octane gasoline.
C.sub.2 -C.sub.4 olefins, e.g., ethylene, propylene and butene, are also valuable organic chemicals which can be used to form polymers. By way of contrast, C.sub.1 -C.sub.3 paraffins (i.e., methane, ethane and propane), particularly in admixture, are less valuable chemicals which are generally used for fuel.
According to U.S. Pat. No. 3,755,486, C.sub.6-10 hydrocarbons undergo dehydrocyclization to benzene and alkylbenzenes in the presence of a Li, Na or K zeolite X or Y or faujasite impregnated with 0.3 to 1.4 percent Pt.
U.S. Pat. No. 3,760,024 discloses the aromatization of a feed containing C.sub.2-4 paraffins and/or olefins in the absence of added hydrogen employing ZSM-5 as catalyst.
According to U.S. Pat. No. 3,855,115, aromatization of hydrocarbons is accomplished employing rhenium-exchanged ZSM-5.
Aliphatic naphthas are upgraded to products of increased aromatics content by the process disclosed in U.S. Pat. No. 3,890,218. The process employs a zeolite catalyst such as ZSM-5 into which one or more metals which increase the aromatization activity of the zeolite, e.g., zinc or cadium, have been incorporated.
In the process disclosed in U.S. Pat. No. 4,347,394 light straight-run naphthas and similar mixtures are converted to highly aromatic mixtures, principally benzene, employing a Group VIII metal-containing intermediate pore size zeolite, e.g. ZSM-5, which has been rendered substantially free of acidity by treatment with an alkali metal compound, e.g., NaOH.
Gaseous feedstocks containing ethane are converted to a mixture of benzene, toluene and xylene ("BTX") in the process of U.S. Pat. No. 4,350,835 utilizing a gallium-containing zeolite such as ZSM-5. A similar catalyst further containing thorium is disclosed in U.S. Pat. No. 4,629,818.
U.S. Pat. No. 4,435,283 describes a method for dehydrocyclizing alkanes employing as catalyst a Group VIII metal-containing large pore zeolite which further contains an alkaline earth metal, e.g., zeolite X, Y or L containing Pt and barium.